Viral expression vectors are widely used to either promote or knockdown the expression of specific genes. In optogenetic studies, viruses can express channelrhodopsin-2 or halorhodopsin to allow optical regulation of neuronal activity. Viruses can also express genetically encoded calcium and voltage indicators to allow optical monitoring of neuronal activity. But current methods have limitations. Here we develop methods for silk-based delivery of adeno-associated virus (AAV) in order to improve the localization of expression, to reduce immunogenic responses and to improve transduction efficiency. Silk is a biocompatible material that when implanted into tissue can dissolve to release viruses. We have found that silk/AAV can be used to express proteins at the tip of an optrode. This leads to alignment with the area of expression and obviates the need of a second surgery to inject AAV. This is simpler than existing methods and promises to increase throughput, lead to more reliable experimental results, and greatly reduce the number of animals and number of experiments required. We will adjust processing conditions to vary the properties of silk films to control the rates of release in order to obtain reliable localized expression and eliminate unwanted expression. We will also determine if silk/AAV reduces inflammatory responses. Injecting AAVs can lead to reactive gliosis, which has been implicated in perturbing synaptic properties. Silk may shield viruses from host immune responses, preventing degradation and improving transduction efficiency. We will compare reactive gliosis and synaptic properties for silk/AAV mixtures and conventional injections of AAV. We will also evaluate the performance of silk/AAV-dependent expression in in vivo imaging (using silk/AAV-coated endoscopes) and optogenetic (using silk/AAV-coated optrodes) studies. Another major goal is to obtain efficient transduction of a large fraction of cells over large regions. We will determine if thin sheets of silk/AAV can b patterned and placed on the surface of the brain to obtaining widespread expression in a defined cortical region. We also propose to use silk/AAV particles to obtain widespread expression. Our preliminary experiments suggest that some formulations of silk/AAV produce stronger and more widespread expression than that produced by injection of virus alone. We will develop approaches to obtain similar expression patterns by implanting small prefabricated silk/AAV particles in the brain. We will assess the utility of silk/AAV in rescue experiments that require widespread expression in a large fraction of a population of cells. We will also determine the utility of silk to express GCaMP in cortical neurons for imaging. If silk can be used to obtain either localized or widespread viral expression it will represent a major technical advance that will make an important contribution to the application of optogenetic approaches and more generally to viral delivery.